Tweaking plant populations is a common approach to try to get more bushels out of each acre, but the potential gains of bumping the number of plants per acre aren't always cut-and-dried, says University of Illinois agronomist and Agriculture.com Corn High Yield Team expert panel member Emerson Nafziger.

"If I'm making a recommendation on plant population response, I'd use overall data. Thirty-five thousand is a pretty good place to be. At 40,000, yields don't respond much," Nafziger says, referencing yield data from tests in central Illinois. "If you want a good blanket recommendation, 35,000 is a pretty good place to be. In southern Illinois, 28,000 is a pretty good place to be. You can go above 30,000 or 35,000 there, just don't expect it to return very much. That changes the mentality a little bit."

The yield response of adjusted plant populations depends a lot on Mother Nature. Last year in Illinois proved that. Dry conditions were especially damaging to the fields where farmers tried higher plant populations. It proves weather should be a major factor to consider when looking at adjusting plant populations, Nafziger says.

"We had some people who put out 50,000 seeds [per acre] this last year who really wish they hadn't," he says. "It was a really bad year to do that -- we had a lot of crop stress, and our roots had trouble getting nutrients."

What about variable-rate seeding? The seeding rate, Nafziger says, should stay within that 5,000-seed window on either side of the optimal population and, more importantly, should be based on solid yield map data in order to be effective.

"Variable-rate maps will be okay. None will be perfect -- the amount you'll be able to swing out of those acres will be minimal," he says. "Variable-rate is pretty dependant on data gathered in a specific year. After-the-fact summaries show correlations, but the predictive ability remains limited."

If you are variable-rate planting this year, make sure you use more than one year's worth of plant population maps, especially in areas that faced extreme weather conditions in 2010. Basing populations on only one year of data can sometimes feed the problems that can rob yields.

"One of the real dangers here is that a farmer farming a field for the first time is looking at a yield map and putting in lower populations in areas that got drowned out, for example, and yielded less. I think some of the best yield maps are the ones the farmers can draw themselves," Nafziger says. "We have to use common sense with these things, not just the data.

"The fact that many planters can do variable-rate and farmers want to use it, they will. The best bet is to set a mid-range population based on response data and economics. From this base, change populations by 800 to 1,000 plants/acre for each 10 bushels in change you expect in yield from the average," he adds.